1. Field of the Invention
The present invention relates to a thermally assisted magnetic recording system and a thermally assisted magnetic recording.
2. Description of the Related Art
In magnetic disk apparatuses which are one type of information recording system mounted on a computer or the like, an increase in recording densities has been required along with an increase in the amount of information to be processed. In order to increase the recording densities of magnetic disk apparatuses, magnetic recording has been increasingly reduced in dimensions, for example, by narrowing a distance between a magnetic disk and a magnetic head, reducing the particle size of magnetic particles constituting a magnetic recording layer of a magnetic recording medium, increasing a coercive force (anisotropy magnetic field) of a magnetic recording medium, or the like.
However, an increase in the magnetic field intensity of a magnetic recording head has a physical limit, which causes the increase in magnetic recording densities to be limited. In high-density recording, it is necessary to reduce the crystal grain size of a magnetic recording medium from the perspective of noise reduction. However, there occurs a problem that the magnetic particles become thermally unstable. Thus, in order to reduce the crystal grain size and also ensure the thermal stability, a magnetic anisotropy energy needs to be increased. The increase in the anisotropy energy, namely, the increase in the anisotropy magnetic field (coercive force) requires the increase in the head magnetic field intensity necessary for recording. However, due to the upper limit of saturation magnetization of a magnetic pole material used for a recording head and the narrowing limit of the distance between a magnetic disk and a magnetic head, it is difficult to further increase the anisotropy magnetic field and the magnetic field intensity from the recording head in the future in proportion as the recording densities become higher.
A magnetic recording method using a patterned recording medium (J. Appl. Phys., 76 6673 (1994), JP Patent Publication (Kokai) No. 2003-45004 or the like) is expected as one of the solutions for the above problems. In the method, a magnetic recording layer of a recording medium is patterned into a recording unit size and magnetic coupling in a pattern is strengthened to enhance durability against thermally magnetization decay in comparison with a conventional magnetic recording method in which a plurality of magnetic particles are employed as a recording unit.
Meanwhile, a thermally assisted magnetic recording technology in which an optical recording technology and a magnetic recording technology are combined has been proposed and is attracting attention. A mechanism for heating a medium is added in a magnetic recording/reproducing head used herein. At the recording time, the magnetic recording/reproducing head generates an application magnetic field and also heats a medium to reduce a medium coercive force. Accordingly, recording is easily performed on a medium having a high coercive force on which recording is difficult to perform by a conventional magnetic head due to an insufficient intensity of a recording magnetic field. The use of an optical head using a near field light as a heat source has been proposed to heat a micro region which is necessary for high-density recording (JP Patent Publication (Kokai) No. 2003-45004 or the like).
Furthermore, a high-density magnetic recording method in which the patterned recording medium is combined with the thermally assisted magnetic recording method has been also proposed (JP Patent Publication (Kokai) No. 2004-355739 or the like). Durability against thermally magnetization decay is higher and further higher densities are easier to obtain also in the thermally assisted magnetic recording method by employing one pattern in which magnetic coupling is strengthened as a recording unit than employing a recording unit constituted by a plurality of magnetic particles.
A mechanism for controlling a recording temperature is required in the thermally assisted magnetic recording. If a recording temperature is low due to a low recording heating power, the coercive force of a recording material is not sufficiently lowered and magnetic recording is not performed. On the other hand, if a recording temperature is high due to an excessive recording heating power, a temperature distribution is expanded and there is a possibility that information that has been already recorded adjacent to a recording position is rewritten. Since a medium heating temperature at the recording time changes depending on an individual difference of heat source element or recording material, variations in frying height when mounted on a slider head, variations in ambient temperature or the like, it is necessary to control the medium temperature at the recording time to have an optimum temperature increase by adjusting the recording heating power. A method of controlling a light intensity by detecting a light intensity from a light source by a photodetector, and suppressing variations in medium heating temperature at the recording time is described in JP Patent Publication (Kokai) No. 2002-133602. Also, JP Patent Publication (Kokai) No. 2002-298301 describes a method of performing trial recording in a specific area and controlling a heating power by comparison of recording data and reproduction data thereby in order to control variations in medium heating temperature at the recording time caused by a difference in linear velocity between the inner periphery and the outer periphery of a disk.